1. Field of the Invention
The present invention relates to a digital processing system for reconstructing recognizable images from surveillance data collected with a synthetic aperture radar (SAR) installed on a satellite or aircraft, and particularly to an image reconstruction method suitable for reconstructing high-quality images at a high processing speed.
2. Description of the Prior Art
In the field of remote sensing using a satellite or aircraft, use is made of SAR which operates in microwave frequencies to provide high resolution images of the ground surface without the influence of clouds. The SAR equipment including a radar sensor and transmission antenna installed on a satellite picks up images of the ground surface from its orbit. Image data sent from the SAR equipment is received by the ground station, in which data is processed to reconstruct images on film and in a magnetic data file.
The following describes in brief the processing of SAR data. More detailed description about SAR is given, for example, in an article entitled "Synthetic Aperture Radar and Remote Sensing Technique" by Ando, Measurement and Control, Vol. 22, No. 2; J. R. Bennett et al "Digital SAR Image Formation Airborne and Satellite Results", Proceedings of 13th International Symposium on Remote Sensing of Environment, pp. 337-348 (1979).
In the received image of SAR, radar echo from a point on the ground surface is distributed with a spread as defined by a point image pattern h(x, y), where x and y represent the range and azimuth directions, and the image cannot be used in its original form. To obtain a ground surface pattern, the received image is first compressed in the range direction, and then compressed in the azimuth direction. The range compression is implemented through a correlation process for each line of image data with point image pattern data. In this case, direct execution of the correlation process takes a lot of time, and therefore the process is sped up using the technique of frequency domain matched filtering with fast Fourier transformation (FFT), complex multiplication and inverse fast Fourier transformation (IFFT). In order to carry out the correlation process using FFT, a point image pattern is generated by computer digital processing, and the FFT of the point image pattern and the FFT of one line of image data are calculated. Since correlation of two sets of data becomes a mere multiplication of the results of the FFT calculation of the data in the frequency domain after the FFT calculation has been made the correlation result for one line of image data is obtained by multiplying the results of the FFT calculation of the above two sets of data and making the IFFT calculation of the result of the multiplication.
A point image pattern in the range compression process is specific to the SAR sensor system and is considered to be completely invariable at least in one scene. On this account, it is sufficient to carry out the point image pattern generation and successive FFT process once for the process of each scene, and this processing time is negligibly small relative to the overall SAR image reconstruction processing time.
The subsequent azimuth compression process is also sped up using the FFT technique, but a point image pattern in this process is dependent on the distance and relative speed between the SAR sensor and the object, and is different for each line. For this reason, the conventional method needs the point image pattern generation and FFT process for each line in the azimuth compression process, which amounts almost 20% the computation for the overall SAR image reconstruction process.